1. Field of the Invention
The invention relates to control devices for contactors, motor starters and the like, which couple electrical power to loads. In particular, the invention concerns a trip indicator for signalling and preferably displaying the trip status of a single or multiple contactor/starter arrangement, remotely from the respective contactor or starter.
2. Prior Art
It is frequently necessary to develop a signal or indicator that represents the status of an electrical contactor or similar device. In a control system having a plurality of functionally related loads, or one or more loads that can be coupled to a power line in different configurations, for example, the status of a contactor or group of contactors may affect whether other contactors, controls, alarms or other devices should be activated or deactivated by associated switching means.
In a more specific example, an AC motor controllable for forward and reverse operation or for operation at different speeds typically is coupled to the power line (e.g., a three phase line) via two or more contactors. The respective contactors couple the motor windings to the line in different configurations as needed for the different functions. In the event that a problem occurs, such as a short circuit, a ground fault, excessive heat build up (thermal overload), phase imbalance or the like, one or both of the contactors will trip, thereby disabling the motor for the selected operation. It is desirable to signal and preferably to display the status of a contactor or starter at a remote location so that appropriate action can be taken or not taken, in view of the detected problem. Such action may involve action by maintenance personnel, in which case a simple and convenient display having indicators for the respective trip modes is advantageous. Preferably the display is located in a convenient place for viewing, which generally means a place remote from the contactor.
Error or trip conditions generally fall into one of several categories, such as short term over-current (i.e., short circuit), long term over-current (thermal overload), ground fault, phase imbalance, etc. A thermal overload trip may occur due to excessive current drawn through one or more windings of a mechanically overloaded motor. A thermal overload condition may build up over a long period of time if a motor is loaded too heavily. The appropriate triggering current setpoint for tripping in the event of a short term over-current is appropriately higher than for a long term overload, the long term setpoint being low and short in time enough to trip promptly in the event of a stalled motor or the like, but not so low or short that the contactor trips when a motor is first started, reversed, etc. in the normal course of operation.
A ground fault trip occurs if current sinking is detected through the system ground. Such a condition is potentially dangerous, and the trip setpoint is low. A phase imbalance trip occurs when there is an improper relationship among the phases of a multi-phase AC system.
An automatic control system that controls a plurality of loads by signalling contactors or that controls one load via one or more contactors advantageously can be made responsive to the trip status of the contactor(s). In a production environment, tripping of a contactor controlling a machine downstream along a process line may be arranged by appropriate remote signalling to trigger contactors upstream in the process to be switched off, or for appropriate action to be taken by production personnel. A control system for effecting related switching may be more or less complex, and may include a relay ladder arrangement, a programmable controller or other device. In a basic control system comprising two or more contactors, the trip signal developed by one of the contactors is typically cascaded through other contactors such that if any of the contactors trips, all the contactors switch off.
This presents a maintenance problem. In the event that a problem occurs such as a short circuit, thermal overload, ground fault or phase imbalance, one or more of the contactors detects the condition, generates an error signal, decouples its load from the line and triggers any cascaded contactors to trip. Often the error signal is also used to generate a bell alarm. Although the system has thereby been protected from damage, and the operators have been alerted, the cause of the error condition still must be determined so that appropriate corrective action can be taken. However, in this basic system as described, the operator may be uncertain as to which contactor generated the error signal, and why.
One alternative is to reset the contactors and hopefully to generate the fault condition again, this time observing operation of the loads and perhaps applying test equipment to isolate the cause of the fault. This is disadvantageous for a number of reasons. The fault condition may be due to an intermittent occurrence, such as brief current overloading when two motors happen to be started or reversed at the same time. Such a fault may not reoccur on resetting the system and may be simply a nuisance trip of no consequence.
On the other hand, the fault condition may be due to an electrical failure of substantial consequence. Recoupling the power in the event of a direct short of the line on the load side of the contactor, for example, could damage the load or the line, or could cause a protective device more proximal to the power source to trip. Recoupling the power in the event of a ground fault or phase imbalance may present a danger of electrocution. For all these reasons, simply resetting the system or any individual contactor is not recommended until the initiating contactor and the reason for the trip are identified, and corrective action is taken if necessary.
It is possible without resetting to determine the nature of an electrical fault and to localize the contactor that most likely initiated a trip, for example using a multimeter or the like to test resistance or continuity among various terminals and ground, in short to verify all the wiring and the condition of the load devices. If such action is taken, and no electrical fault is found, uncertainty remains as to whether a nuisance fault occurred and can be ignored, or whether a real problem exists but was inadvertently overlooked. Testing a system sufficiently that the operator is confident can be difficult and time consuming.
It would be possible to provide additional instrumentation, mechanical devices, memory storage elements or similar means associated with the contactors that enable a technician to determine the location and cause of a trip condition. It would also be possible to provide metering elements and/or recording devices that can be checked to determine the operating parameters of the power distribution system and the load, allowing the source and type of fault to be deduced. These alternatives are expensive. Moreover, as a practical matter, the operator may be willing to take the chance that a trip was a nuisance trip rather than a fault of consequence, especially if nuisance trips are not unusual, rather than to take the time to ring out the circuits, to check any instrumentation, recording elements or the like, or even to open the cabinets housing the starters to view any mechanical or electrical indicators therein. What is needed is an inexpensive and convenient means for generating and storing an indication of the source and type of trip, at a location remote from the contactor or motor starter, and preferably without adding any substantial circuitry or instrumentation to the standard contactor.
Some systems provide a status display associated with a controller arranged to communicate with one or more contactors. The display is operable to indicate the contactor status, and therefore the load circuit status, of associated contactors. The Westinghouse Electric Corporation ADVANTAGE.TM. line of control modules provide a status display of this description, the display comprising colored LEDs. The status light colors are chosen such that normal operations such as "run" use green indicators while trip indicators such as thermal overload, or phase imbalance use red. In one arrangement of the ADVANTAGE control module, blinking of particular lights that normally are used to indicate "run," "stop" and similar modes, or trip status, are blinked to signal the operational status of up to four associated contactors. A display of this type on a control module is helpful to localize the source of a trip, at least to the level of the group of contactors coupled to the control module. The control module comprises a microprocessor that tests the operability of communications with the contactors in addition to controlling the contactors to achieve coordinated operation. The controller is typically located inside a cabinet adjacent the contactors it services.
It would facilitate maintenance procedures to provide trip status indicators in locations remote from the controller. In different applications, the starters or contactors and their controller may located together or apart from one another and from the loads being controlled. In order for a technician to determine the cause of a trip, the appropriate controller must be located before the trip status can be read.
For safety and practical reasons, it would be advantageous to provide means such as an indicator circuit, responsive to a trip status of a contactor or the like, particularly for use with contactors coupled to a control module such as the ADVANTAGE control module. It would furthermore be desirable that such a device be inexpensive and simple, providing a means for indicating or otherwise responding to a trip source and type, that can be placed remote from the contactor circuits, and does not require the addition of extensive supporting circuitry, power supplies and logic elements.